How to mirror a walk cycle?
You may use the Paste mirrored keyframes from buffer button.


In 3D View window select all bones with A (or select only the ones you want to mirror). Do the same in the Dope Sheet Editor. Press the Copy selected keyframes to the buffer button (or Ctrl+C, while being in the Dope Sheet Editor), go to the frame where your half-side animation ends and press the Paste mirrored keyframes from buffer (or Shift+Ctrl+V).